Piston/combustion chamber configurations for enhanced ci engine performace

ABSTRACT

Piston face ( 104, 204, 304 ) and combustion chamber ( 18 ) designs for use particularly in HSDI (high speed direct injection) diesel engines include an open bowl ( 108 208, 308 ) characterized by a large face perimeter region ( 106, 206, 306 ) on the piston face ( 104, 204, 304 ), and a bowl ( 18 ) defined by a first depressed region ( 112, 212, 312 ) gently sloping radially inwardly from the face perimeter region ( 106, 206, 306 ) and a second depressed region ( 116, 216, 316 ) sharply sloping radially inwardly from the first depressed region ( 112, 212, 312 ) to the bowl floor ( 120, 220, 320 ). Injection is preferably directed towards an intermediate edge which is well-defined between the first and second depressed regions, resulting in portions of the injected fuel plume being directed to both the squish regions and the portion of the bowl situated below the intermediate edge. The designs promote premixed or MK (Modulated Kinetics) combustion, with a concomitant reduction in soot and nitrous oxides (NOx) emissions while maintaining or enhancing brake specific fuel consumption.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC §119(e) to U.S.Provisional Patent Application 60/387,865 filed 11 Jun. 2002, theentirety of which is incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with United States government support awarded bythe following agencies:

U.S. Department of Energy (DOE) Grant No. DE-FG04-99AL66269 The UnitedStates has certain rights in this invention.

FIELD OF THE INVENTION

This disclosure concerns an invention relating generally to pistonand/or combustion chamber configurations which allow reduction ofemissions and fuel consumption in internal combustion engines, and morespecifically to piston and/or combustion chamber configurations whichprovide emissions reduction in compression ignition (CI or diesel)engines.

BACKGROUND OF THE INVENTION

Common pollutants arising from the use of compression ignition (CI ordiesel) internal combustion engines are nitrogen oxides (commonlydenoted NO_(x)) and particulates (also known simply as “soot”). NO_(x)is generally associated with high-temperature engine conditions, and maybe reduced by use of measures such as exhaust gas recirculation (EGR),wherein the engine intake air is diluted with relatively inert exhaustgas (generally after cooling the exhaust gas). This reduces the oxygenin the combustion region and obtains a reduction in maximum combustiontemperature, thereby deterring NO_(x) formation. Particulates include avariety of matter such as elemental carbon, heavy hydrocarbons, hydratedsulfuric acid, and other large molecules, and are generally associatedwith incomplete combustion. Particulates can be reduced by increasingcombustion and/or exhaust temperatures, or by providing more oxygen topromote oxidation of the soot particles. Unfortunately, measures whichreduce NO_(x) tend to increase particulate emissions, and measures whichreduce particulates tend to increase NO_(x) emissions, resulting in whatis often termed the “soot-NO_(x) tradeoff”.

At the time of this writing, the diesel engine industry is facingstringent emissions legislation in the United States, and is strugglingto find methods to meet government- imposed NO_(x) and soot targets forthe years 2002-2004 and even more strict standards to be phased instarting in 2007. One measure under consideration is use of exhaustafter-treatment (e.g., particulate traps) for soot emissions control inboth heavy-duty truck and automotive diesel engines. However, in orderto meet mandated durability standards (e.g., 50,000 to 100,000 miles),the soot trap must be periodically regenerated (the trapped soot must beperiodically re-burned). This requires considerable expense andcomplexity, since typically additional fuel must be mixed and ignited inthe exhaust stream in order to oxidize the accumulated particulatedeposits.

Apart from studies directed to after-treatment, there has also beenintense interest in the more fundamental issue of how to reduce NO_(x)and particulates generation from the combustion process and therebyobtain cleaner “engine out” emissions (i.e., emissions directly exitingthe engine, prior to exhaust after-treatment or similar measures). Moststudies in this area relate to timing the fuel injection, tailoring theinjection rate during injection so as to meet desired emissionsstandards (including the use of split or multiple injections), modifyingthe mode of injection (e.g, modifying the injection spray pattern),premixing of fuel and air, and shaping combustion chambers.

One promising field of study has related to the so-called premixed orModulated Kinetics (MK) combustion mode, which is primarilycharacterized by three events: (1) injection is made at or near top deadcenter; (2) the ignition delay exceeds the injection duration so thatthe fuel/air mixture is at least partially premixed prior to combustion;and (3) a leaner-than-usual fuel/air mixture is used. The object is tominimize the diffusion burning which drives standard diesel combustionand emissions formation, wherein oxidant (fuel) is provided to theoxidizer (air) with mixing and combustion occurring simultaneously. Indiffusion burning, fuel droplets within an injected spray plume have anouter reaction zone surrounding a fuel core which diminishes in size asit is consumed, and high soot production occurs at the high-temperature,fuel-rich spray core. In contrast, premixed burning thoroughly mixesfuel and air prior to burning, resulting in less soot production andalso deterring the high-temperature diffusion flame region which spawnsexcessive NOx. One difficulty with achieving premixed combustion is thedifficulty in controlling all variables needed for its achievement,especially across a wide range of operating speeds and loads.

Combustion chamber geometry is an interesting field of study because itis one of the few variables critical to engine performance that remainsforever fixed once it is initially chosen. Additionally, it is one ofthe few variables that is relatively cost-tolerant: manufacturing onechamber configuration generally does not have significant costdifference from manufacturing a different configuration (barringunusually complex designs). Combustion chamber studies have largelyfocused on the shape of the piston face since most diesel engines use aflat (or nearly flat) cylinder head opposite the piston face, and it iswell known that the geometry of the piston bowl (the depressionconventionally formed on the piston face) has a significant influence onthe diesel combustion process. However, the optimization of chamberconfigurations (for enhanced engine performance is often more a matterof art than science. Owing to the number of variables involved in engineperformance, and the interaction between these variables, the effect ofdifferent chamber configurations is not easily predicted. Nevertheless,some basic trends in chamber design can be identified.

In direct injection (DI) diesel engines (i.e., engines wherein the fuelis directly injected into the combustion chamber, as opposed to anindirect injection scheme wherein fuel is injected into a pre-chamberopening onto a main combustion chamber adjacent the piston), mostpresent combustion chamber designs can be categorized as either are-entrant chamber design or an open chamber design. A reentrant designutilizes a piston bowl which curves inwardly from the bowl's top edgestoward the sides of the piston to enhance mixing via swirl (preliminary)currents, which are primarily generated from the intake air flow (thoughsquish or secondary currents, which are primarily generated by forcingair off of the piston face into the bowl as the piston face approachesthe cylinder head, may also contribute to mixing). An open design lackssuch inwardly-extending edges, and instead relies more on fuel spray toprovide the desired mixing. Most HSDI (high speed direct ignition)diesel engines, such as automotive engines, achieve the desired degreeof mixing by using a small diameter, relatively deep, re-entrant typepiston bowl. In contrast, larger heavy-duty engines, which operate atlower speeds (and thus can utilize lower mixing rates), typically uselarger diameter, open-type bowls. While fuel spray orientation varies,fuel spray for reentrant bowls is generally oriented towards the bowllip, where it is pulled into the bowl by swirl currents. In open bowls,the fuel spray is generally oriented towards the bottom surface of thebowl or towards the squish region (the region on the piston facebounding the bowl).

Studies have indicated that re-entrant chamber designs generally resultin better fuel economy and lower emissions in HSDI engines. Middlemiss(1978) found that re-entrant designs provide higher mixing rates,thereby allowing retarded injection timings and higher speed operation(Middlemiss, I. D., “Characteristics of the Perkins ‘Squish Lip’ DirectInjection Combustion System”, SAE 780113, 1978). This results in lowersoot and NO_(x) emissions, with no degradation in fuel economy. Saito etal. (1986) also found that a re-entrant chamber produces shorterignition delays, lower fuel consumption, and lower soot and NO_(x)emissions when used with retarded injection timings (Saito, T.,Yasuhiro, D., Uchida, N., Ikeya, N., “Effects of Combustion ChamberGeometry on Diesel Combustion”, SAE 861186, 1986). Later studies havesuggested that the use of a centrally-situated cone, frustum, or otherraised “crown” within the bowl may also have a beneficial effect onperformance and emissions (e.g., Zhang, L., Ueda, T., Takatsuki, T.,Yokota, K., “A Study of the Effects of Chamber Geometries on FlameBehavior in a DI Diesel Engine”, SAE 952515, 1995). Other studiessuggest that it is necessary to consider injection spray angle andinjection timing along with chamber geometry, since these threevariables strongly interact to determine engine performance (e.g., DeRisi, A., Manieri, D., and Laforgia, D., “A Theoretical Investigation onthe Effects of Combustion Chamber Geometry and Engine Speed on Soot andNOx Emissions”, ASME-ICE, vol. 33-1, pp. 51-59, Book No. G1127A, 1999.)

While prior studies have resulted in improvements in engine performance,there is still significant room for improvement in combustion chamberdesigns which result in reduced emissions with reasonable BSFC (brakespecific fuel consumption, i.e., fuel consumption per unit of usefuloutput power).

SUMMARY OF THE INVENTION

The invention, which is defined by the claims set forth at the end ofthis document, is directed to methods and apparata which provide pistondesigns (and therefore combustion chamber designs) which result insignificant emissions reduction in HSDI engines while maintaining orreducing BSFC. A piston and combustion chamber in accordance with theinvention includes a piston face bounded by a piston side, with a faceperimeter region extending inwardly from the piston side and preferablybeing oriented at least substantially perpendicular to the piston side.An open bowl descends from the face perimeter region, with the bowlincluding a first depressed region descending from the face perimeterregion at a first angle (the first angle being measured with respect tothe face perimeter region); a second depressed region descending fromthe first depressed region at a second angle which is greater (i.e.,steeper) than the first angle (the second angle also being measured withrespect to the face perimeter region); and a bowl floor extending fromthe second depressed region, preferably across the center of the piston.The first angle at which the first depressed region descends from theface perimeter region is preferably acute, more preferably less than 30degrees, whereas the second angle at which the second depressed regionis preferably greater than 45 degrees. The face perimeter region ispreferably rather large (e.g., occupying 40% or more of the piston face,as measured from a plane perpendicular to the axis of the piston) so asto define a relatively large squish region within the combustionchamber. Additionally, it is also preferred that a re-entrant bowldesign be avoided, i.e., the first and second depressed regions do notslope outwardly towards the piston side as they extend downwardlytowards the bowl floor.

The piston travels within a cylinder to define the combustion chamberbetween the piston face and the cylinder head of the cylinder. A fuelinjector is situated within the combustion chamber, and is configured toinject a fuel plume along a direction oriented above the bowl floor andbelow the face perimeter region, more preferably toward the firstdepressed region and at or adjacent to an intermediate edge definedbetween the first and second depressed regions.

Simulations and experiments have demonstrated that piston and combustionchamber designs having the foregoing characteristics are able to attaindecreased emissions while maintaining or reducing BSFC. Furtheradvantages, features, and objects of the invention will be apparent fromthe following detailed description of the invention in conjunction withthe associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an exemplary combustion chamber 18 showinga particularly preferred configuration for a piston face 104, with thepiston 100 being situated within its cylinder (including cylinder walls10 and cylinder head 12) at top dead center (i.e., with the piston face104 being shown at its closest distance to the cylinder head 12 duringoperation), and showing a fuel spray plume 20 being ejected frominjector 16.

FIG. 2 illustrates the profile of the preferred configuration for pistonface 104 (as also shown in FIG. 1) along a plane coincident with thecentral axis of the piston 100.

FIG. 3 illustrates the profile of another preferred configuration for apiston face 204 along a plane coincident with the central axis of thepiston 200.

FIG. 4 illustrates the profile of another preferred configuration for apiston face 304 along a plane coincident with the central axis of thepiston 300.

DETAILED DESCRIPTION OF THE INVENTION

Preferred versions of the piston and combustion chamber designs of theinvention will now be described with reference to the piston faceconfigurations of FIGS. 2-4, any of which may be utilized in a dieselengine cylinder and combustion chamber such as the one illustrated inFIG. 1 (which utilizes a piston 100 having the piston face configurationin FIG. 2). The cylinder is defined by cylinder walls 10 along which thepiston 100 slides, with the piston having a piston side 102 surroundinga piston face 104. During engine operation, the piston face 104alternately approaches and retreats from the cylinder head 12, whereinintake and exhaust valves 14 are provided along with an injector 16. Thespace between the piston face 104, cylinder walls 10, and cylinder head12 defines the combustion chamber 18 wherein the combustion event occursafter the injector 16 injects a fuel plume 20 into the combustionchamber 18. Note in FIG. 1, the injector 16 is shown injecting one fuelplume 20 into the combustion chamber 18 at an angle (as opposed to alonga direction oriented generally coaxially with the axis of the piston100), though the injector 16 is not shown oriented along this angle (aswould usually be the case in practice). Typically, HSDI diesel enginefuel injectors feature multiple spray plumes that originate from 4-10holes in the injector fuel spray nozzle tip. In this respect, it shouldbe understood that FIG. 1 depicts an exemplary idealized cylinder, andthe piston 100 and combustion chamber 18 designs described below may beimplemented in engines having cylinder configurations radicallydifferent than the one shown.

The following piston 100 and combustion chamber designs are particularlysuitable for use in HSDI (high speed direct injection) diesel engineswhich primarily operate at medium speed and part load, with singleinjection. HSDI engines may be generally characterized as automotivediesel engines which operate at speeds up to approximately 4500 rpm, andwhich generally have a 7-10 cm cylinder bore and approximately 0.51displacement per cylinder; additionally, HSDI engines generally usecentral injection (i.e., a single multi-hole injector is situated at orabout the central axis of the cylinder).

All of the piston designs illustrated in FIGS. 1-4 will now be generallydescribed in terms of their common characteristics, with particularreference being made to the particularly preferred design of FIGS. 1 and2. The piston face 104 includes a face perimeter region 106 whichextends radially inwardly from the surrounding piston side 102, andwhich is preferably oriented at least substantially perpendicular to thepiston side 102 (or more precisely, which is preferably orientedsubstantially parallel to the overall plane of the opposing surface ofthe cylinder head 12 so that a squish region of uniform depth is formedabout the circumference of the combustion chamber 18). A bowl 108descends from the face perimeter region 106 at a face region edge 110,and includes a first depressed region 112 descending radially inwardlyfrom the face region edge 110 of the face perimeter region 106 to anintermediate edge 114, a second depressed region 116 descending radiallyinwardly from the intermediate edge 114 of the first depressed region112 to a bowl floor edge 118, and a bowl floor 120 which then extendsradially inwardly from the second depressed region 116 and bowl flooredge 118 across the center of the piston face 104.

The bowl 108 is of the open type rather than the re-entrant type, i.e.,the surfaces between the face perimeter region 106 and the bowl floor120 do not slope outwardly towards the piston side 102 as they extenddownwardly towards the bowl floor 120. The use of an open design ratherthan a re-entrant design is somewhat uncommon for HSDI engines, but aswill be discussed later, the open design appears to generate superiorengine performance. The first depressed region 112 descends gently fromthe face perimeter region 106 at a first angle, and the second depressedregion 116 steeply descends from the first depressed region 112 at agreater second angle (with both the first and second angles beingmeasured with respect to a plane perpendicular to the axis of the piston100). Since the first depressed region 112 need not necessarily take aplanar form, i.e., its angle with respect to the face perimeter region106 may vary along a length of the first depressed region 112 (suchlength being measured radially from the axis of the piston 100), it isuseful to regard the first angle as being measured from the faceperimeter region 106 along a line defined between the edges of the firstdepressed region 112 (i.e., between the face region edge 110 and theintermediate edge 114). Similarly, the second depressed region 116 neednot necessarily take a planar form, and it is useful to regard thesecond angle as being measured from the plane of the face perimeterregion 106 along a line defined between the edges of the seconddepressed region 116 (i.e., between the intermediate edge 114 and thebowl floor edge 118). Preferably, the first depressed region 112descends from the face perimeter region 106 at an acute first angle ofless than 30 degrees, and the second depressed region 116 descends fromthe first depressed region 112 at a second angle of greater than 45degrees.

The piston face 102 is also somewhat unusual as compared to most currentHSDI engines in that it has a large squish volume (i.e., it has a largevolume situated outside the bowl 108 and above the face perimeter region106 at top dead center). Preferably, the face perimeter region 106occupies at least 40% of the area of the piston face 104, as measuredfrom projection of the face perimeter region 106 onto a planeperpendicular to the axis of the piston 100. The first depressed region112, which might be expected to contribute to the squish current effectsgenerated by the face perimeter region 106 since it is only slightlydepressed from the face perimeter region 106, also occupies a relativelylarge portion of the piston face 104. Preferably, it occupies between15%-30% of the area of the piston face 104, as measured from aprojection of the first depressed region 112 onto a plane perpendicularto the axis of the piston 100.

Turning now to a discussion of the specific characteristics of each ofthe piston and combustion chamber designs of FIGS. 1-4, in the pistonface 104 of FIG. 2, the face perimeter region 106 and bowl 108 haveapproximately the same area (as measured from a projection onto a planeperpendicular to the axis of the piston 100), with the face perimeterregion 106 occupying slightly over 50% of the area of the piston face.The first depressed region 112 occupies approximately 25% of the area ofthe piston face 104, and the bowl floor 120 occupies approximately 15%of the area of the piston face 104, when measured along the same plane.The first depressed region 112 gently descends from the face perimeterregion 106 at a first angle of approximately 20 degrees with respect tothe face perimeter region 106, and defines approximately 30% of thedepth of the bowl 108 (as measured from the plane of the face perimeterregion 106 to the plane of the bowl floor 120). The second depressedregion 116 steeply descends from the first depressed region 112 at asecond angle of approximately 75 degrees with respect to the plane ofthe face perimeter region 106, and defines approximately 70% of thedepth of the bowl 108 (as measured from the plane of the face perimeterregion 106 to the plane of the bowl floor 120).

In the piston face 204 of FIG. 3, the face perimeter region 206 issignificantly larger than the bowl 208, and occupies approximately 70%of the area of the piston face 204 (as measured from a projection onto aplane perpendicular to the axis of the piston 200). The first depressedregion 212 occupies approximately 20% of the area of the piston face204, and the bowl floor 220 occupies approximately 5% of the area of thepiston face 204, when measured along the same plane. The first depressedregion 212 gently descends from the face perimeter region 206 at a firstangle of approximately 35 degrees with respect to the face perimeterregion 206, and defines approximately 40% of the depth of the bowl 208(as measured from the plane of the face perimeter region 206 to theplane of the bowl floor 220). The second depressed region 216 steeplydescends from the first depressed region 212 at a second angle ofapproximately 50 degrees with respect to the face perimeter region 206,and defines approximately 60% of the depth of the bowl 208 (as measuredfrom the plane of the face perimeter region 206 to the plane of the bowlfloor 220). A raised crown 222 is centrally located on the bowl floor220, but it is relatively low and extends upwardly no further than about15% of the depth of the bowl 208.

In the piston face 304 of FIG. 4, the face perimeter region 306 issmaller than in the prior embodiments, and occupies slightly over 40% ofthe area of the piston face 304 (as measured from a projection onto aplane perpendicular to the axis of the piston 300). The first depressedregion 312 occupies approximately 25% of the area of the piston face304, and the bowl floor 320 occupies approximately 20% of the area ofthe piston face 304, when measured along the same plane. The firstdepressed region 312 gently descends from the face perimeter region 306at a first angle of approximately 10 degrees with respect to the faceperimeter region 306, and defines approximately 33% of the depth of thebowl 308 (as measured from the plane of the face perimeter region 306 tothe plane of the bowl floor 320). The second depressed region 316steeply descends from the first depressed region 312 at a second angleof approximately 50 degrees with respect to the face perimeter region306, and defines approximately 66% of the depth of the bowl 308 (asmeasured from the plane of the face perimeter region 306 to the plane ofthe bowl floor 320).

The foregoing combustion chamber designs are preferably used with aninjector which injects its fuel plumes 20 along a direction orientedabove the bowl floors 120, 220, and 320 and below the face perimeterregions 106, 206, and 306, preferably so that the fuel plume 20 isoriented along an axis directed closer to the intermediate edges 114,214 and 314 than to the bowl floors 120, 220 or 320 or the faceperimeter regions 106, 206, or 306. Most preferably, the fuel plume 20is oriented toward the first depressed regions 112, 212, and 312 andadjacent to the intermediate edges 114, 214 and 314. In simulations,this fuel plume orientation is found to split the fuel vapor between thebowls 108, 208 and 308 and the squish regions situated above the faceperimeter regions 106, 206, and 306.

Results from performance simulations of the various piston andcombustion chamber configurations of FIGS. 1-4 at medium speed and partload are provided in the accompanying TABLE 1. The piston 100 of FIGS. 1and 2 resulted in exceptionally low emissions with admirable brakespecific fuel consumption. The piston 200 of FIG. 2 had slightly lessadvantageous (though still good) results, with soot production and BSFCbeing somewhat higher. The piston 300 of FIG. 3 had the leastadvantageous performance of the three designs, with exceptionally lowsoot production but higher NOx and BSFC. Exhaust gas recirculation wasused in all cases to attain better emissions. The pistons 100 and 200demonstrate the characteristics of premixed or Modulated Kinetics (MK)combustion, which (as discussed previously) is known to result inreduced emissions, but which is often difficult to achieve. TABLE 1Performance characteristics of designs in FIGS. 1-4 Parameter SOI (Startof Injection, crank angle) +1 +1 +13 DOI (Duration of Injection, 17.8817.88 18.77 crank angle) Swirl Ratio 3.3 3.3 3.3 % EGR (Exhaust GasRecirculation) 19.5 15.6 19.5 CR (Compression Ratio) 14.76 15.72 15.72Soot (g/kg-fuel) 0.656 1.345 0.27 NOx (g/kg-fuel) 0.696 0.693 3.89 BSFC(Brake Specific Fuel 254 272 352 Consumption, g/kW-hr)

Further details on the foregoing versions of the invention (and otherversions as well) can be found in the paper Wicknan, D. D., Yun, H.,Reitz, R. D., “Split-Spray Piston Geometry Optimized for HSDI DieselEngine Combustion”, SAE 2003-01-0348, 2003, the entirety of which isincorporated by reference herein.

The various preferred versions of the invention are shown and describedabove to illustrate different possible features of the invention and thevarying ways in which these features may be combined. Apart fromcombining the different features of the different versions in varyingways, other modifications are also considered to be within the scope ofthe invention. Following is an exemplary list of such modifications.

The piston face profiles depicted in FIGS. 1-4 should be consideredrepresentative of piston faces 104, 204, and 304 which are is axiallysymmetric about the axis of their pistons (i.e., the profiles of FIGS.1-4, when rotated about their central axes, define the contours of thepiston faces 104, 204, and 304). However, it should be understood thatthe pistons 100, 200, and 300 need not necessarily be axisymmetric; forexample, the face perimeter regions, first depressed regions, and seconddepressed regions need not each have a uniform radial length as theyextend about the piston face, and/or sections of the face perimeterregions, first depressed regions, and second depressed regions may havenegligible radial length (e.g., the face perimeter region might beformed to extend from at least a substantial portion of the piston side,but may have negligible radial length at certain sections so that thefirst depressed region extends directly from the piston side).

While the foregoing piston and combustion chamber designs have beendescribed as being particularly suitable for use in HSDI engines, thedesigns may also be beneficial for use in larger engines (e.g., truckand medium-speed locomotive engines). It is also expected that thedesigns are also beneficially used at other speeds and loads, and withsplit (multiple) injections.

The invention is not intended to be limited to the preferred embodimentsdescribed above, but rather is intended to be limited only by the claimsset out below. Thus, the invention encompasses all alternate embodimentsthat fall literally or equivalently within the scope of these claims.

1. A diesel combustion chamber comprising: a. a piston having a pistonface bounded by a piston side, wherein the piston face comprises: (1) aface perimeter region extending radially inwardly from the piston side;(2) an open bowl descending from the face perimeter region, the bowlincluding: i. a first depressed region descending radially inwardly fromthe face perimeter region at a first angle, the first angle beingmeasured with respect to the face perimeter region; ii. a seconddepressed region descending radially inwardly from the first depressedregion at a second angle which is greater than the first angle, thesecond angle being measured with respect to the face perimeter region;and iii. a bowl floor extending radially inwardly from the seconddepressed region; b. a cylinder wherein the piston travels, the cylinderincluding a cylinder head opposite the piston face, whereby a combustionchamber is defined between the piston face and the cylinder head, and c.an injector situated within the combustion chamber, the injector beingcapable of injecting a fuel plume into the combustion chamber; whereinthe injector injects the fuel plume along a direction oriented above thebowl floor and below the face perimeter region.
 2. The diesel combustionchamber of claim 1 wherein the first depressed region descends from theface perimeter region at a first angle of less than 30 degrees.
 3. Thediesel combustion chamber of claim 1 wherein the second depressed regiondescends from the first depressed region at a second angle of greaterthan 45 degrees.
 4. The diesel combustion chamber of claim 1 wherein theface perimeter region occupies at least 40% of the piston face, asmeasured from a plane perpendicular to the axis of the piston.
 5. Thediesel combustion chamber of claim 1 wherein the injector injects thefuel plume along a direction oriented toward the first depressed regionand adjacent to the intermediate edge.
 6. The diesel combustion chamberof claim 1 wherein the injector injects the fuel plume along a directionoriented toward the intermediate edge.
 7. The diesel combustion chamberof claim 1 wherein the piston face is axially symmetric about the axisof the piston.
 8. A diesel combustion chamber comprising: a. a pistonhaving a piston face bounded by a piston side, wherein the piston facecomprises: (1) a face perimeter region extending from at least asubstantial portion of the piston side, wherein the face perimeterregion is oriented at least substantially perpendicular to the pistonside; (2) a first depressed region gently descending from the faceperimeter region at a first angle with respect to the face perimeterregion; (3) a second depressed region steeply descending from the firstdepressed region at a second angle with respect to the face perimeterregion, thereby defining an intermediate edge between the first andsecond depressed regions; and (4) a bowl floor extending from the seconddepressed region and extending across the center of the piston face; b.a cylinder wherein the piston travels, the cylinder including a cylinderhead opposite the piston face, whereby a combustion chamber is definedbetween the piston face and the cylinder head, and c. an injectorsituated within the combustion chamber, the injector being capable ofinjecting a fuel plume into the combustion chamber; wherein the injectorinjects the fuel plume along a direction oriented above the bowl floorand below the face perimeter region.
 9. The diesel combustion chamber ofclaim 8 wherein the surfaces of the first and second depressed regionsdo not slope outwardly towards the piston side as they extend downwardlytowards the bowl floor.
 10. The diesel combustion chamber of claim 8wherein: a. the first angle is acute; and b. the second angle is greaterthan the first angle.
 11. The diesel combustion chamber of claim 8wherein the piston face is axially symmetric about the axis of thepiston.
 12. The diesel combustion chamber of claim 8 wherein the faceperimeter region occupies at least 40% of the piston face, as measuredfrom a plane perpendicular to the axis of the piston.
 13. The dieselcombustion chamber of claim 8 wherein the first depressed regiondescends from the face perimeter region at a first angle of less than 30degrees.
 14. The diesel combustion chamber of claim 8 wherein the seconddepressed region descends from the first depressed region at a secondangle of greater than 45 degrees.
 15. The diesel combustion chamber ofclaim 8 wherein the injector injects the fuel plume along a directionoriented closer to the intermediate edge than to the bowl floor or theface perimeter region.
 16. The diesel combustion chamber of claim 8wherein the injector injects the fuel plume along a direction orientedtoward the first depressed region.
 17. A diesel combustion chambercomprising: (1) a piston, (2) a cylinder wherein the piston travels, thecylinder including a cylinder head opposite the piston face, whereby acombustion chamber is defined between the piston face and the cylinderhead, and (3) an injector situated within the combustion chamber, theinjector being capable of injecting a fuel plume into the combustionchamber, wherein the piston includes a piston face bounded by a pistonside, the piston face having: a. a face perimeter region extendinginwardly from the piston side; b. a first depressed region descendingfrom the face perimeter region at a first angle with respect to the faceperimeter region; c. a second depressed region descending from the firstdepressed region at a second angle with respect to the face perimeterregion, the second angle being greater than the first angle; and d. abowl floor extending from the second depressed region and extendingacross the center of the piston, wherein the injector injects the fuelplume along a direction oriented toward the first depressed region andat or adjacent to the intermediate edge.
 18. The diesel combustionchamber of claim 17 wherein the first depressed region, second depressedregion, and bowl floor define an open bowl in the piston face.
 19. Thediesel combustion chamber of claim 17 wherein the surfaces of the firstand second depressed regions do not slope outwardly towards the pistonside as they extend downwardly towards the bowl floor.